1. Field of the Invention
The present invention relates to method for driving a plasma display panel and more particularly to method of driving an AC discharge memory-type plasma display panel in which sustaining charge erasing operations are incorporated.
2. Description of the Related Art
In general, a plasma display panel (hereinafter referred to as PDP) is featured by thin construction, no flicker on the display and a great display contrast ratio. Moreover, it has various features including a large screen, high response speed, spontaneous light emission as well as multi-color emission. Owing to these characteristics, the plasma display panel is widely used for display devices and color imaging displays in the field of computers and related equipment.
The PDP can be classified into two types, one being of an ac discharge type adapted to operate in the ac discharge state using an electrode coated with dielectrics, the other being of a DC (Direct Current) discharge type adapted to operate in the DC discharge state with an electrode exposed to discharge gas space. The ac discharge type PDP can be further sub-classified into two types, one being of a memory-type utilizing a driving method using memory functions of a discharge cell and the other being of a refresh type not using such memory functions of the discharge cell. Luminance of the PDP is proportional to the number of discharging operations, i.e., the number of repetition of the pulse voltage. In the case of the above refresh type PDP, if display capacity becomes large, luminance is reduced and therefore it is mainly used in the PDP with small display capacity.
FIG. 11 is a cross-sectional diagram illustrating configurations of one display cell 16 of an AC discharge memory-type PDP. This display cell 16 is comprised of a front insulating board 1 and a rear insulating board 2 both of which are made of glass, trace electrodes 5 and 6 adapted to lie on a scanning electrode 3 and a sustaining electrode 4 in order to lower resistance of the electrode, a data electrode 7 formed, on the rear insulating board 2, so that it may intersect at right angles with the scanning electrode 3 and the sustaining electrode 4, discharge gas space 8 filled with discharge gas including helium, neon, xenon or their mixed gas disposed in space between the front insulating board 1 and the rear insulating board 2, a phosphor 11 used to convert ultra violet rays generated by discharge of the discharge gas to visible light 10, a dielectric layer 12 used to coat the scanning electrode 3 and the sustaining electrode 4 therewith, a protecting layer 13 composed of magnesium oxide or the like to protect the dielectric layer against discharging and a dielectric layer 14 used to coat the data electrode therewith.
Next, operations of discharge of a selected display cell 16 are hereafter described by referring to FIG. 11. If discharge is allowed to occur by applying a pulse voltage exceeding discharge threshold values between the scanning electrode 3 and the data electrode 7, positive and negative charges are attracted to surfaces of dielectric layers disposed on both sides in response to polarity of the pulse voltage, causing charges to be accumulated.
Since the equivalent voltage caused by accumulation of electric charges, i.e., wall voltages are of the opposite polarity, an effective voltage within the cell is lowered as the discharge grows and, even if the above pulse voltage is maintained at a definite value, the discharge cannot be maintained and stops in the end.
After that, if a sustaining discharge pulse being a pulse voltage of the same polarity as a wall voltage is applied between the scanning electrode 3 and the sustaining electrode 4 being adjacent to each other, since wall charges are superposed on as the effective voltage, even if the amplitude of the voltage of the sustaining discharge pulse is low, the discharge is possible at a voltage exceeding the discharge threshold. Therefore, the discharge can be maintained by alternately applying a sustaining discharge pulse between the scanning electrode 3 and the sustaining electrode 4. This function is a memory function described above.
The above sustaining discharge can be stopped by applying, to the scanning electrode 3 or the sustaining electrode 4, an erasing pulse having wide pulse width and a low voltage or an erasing pulse having a mild fall (or a rise), a wide width and almost the same voltage as that of the sustaining pulse that can be used to neutralize wall charges or an erasing pulse having narrow width and the almost the same voltage as that of the sustaining pulse or a pulse combined with these pulses.
FIG. 12 is a top plan view of approximate configurations of the PDP composed of display cells 16 disposed in a matrix state shown in FIG. 11. The PDP 15 is a panel used for dot matrix display in which "m.times.n" pieces of lines and rows are arranged. The scanning electrodes Ss1, Ss2, . . . Ssm and the sustaining electrode Su disposed in parallel with each other are arranged as line electrodes. The data electrodes D1, D2, . . . Dn disposed so as to intersect, at right angles, with the scanning and sustaining electrodes as row electrodes.
In FIG. 13, Wu represents a sustaining electrode driving pulse supplied to a sustaining electrode Su, Ws1, Ws2, . . . Wsm are driving pulses supplied to each of scanning electrodes Ss1, Ss2, . . . Ssm, and Wd is a data electrode driving pulse supplied to a data electrode Di (1.ltoreq.i.ltoreq.n). One cycle (one frame) of driving contains a pre-discharged period, a writing period, a sustaining discharge period and a sustaining charge erasing period and a desired image can be obtained by repeating this cycle of driving.
The pre-discharge period is a period to generate active particles and wall charges in the discharge gas space in order to obtain stable writing discharge characteristics during the writing period. In the operations, all the pre-discharge pulses Pp+ and Pp- are applied and then further the pre-discharge erasing pulse Ppe used to discharge all display cells 16 of the PDP 15 is applied to all the scanning electrodes in order to erase electric charges, out of wall charges generated during the pre-discharge period, to interfere with the writing discharge and sustaining discharge. That is, the pre-discharge pulse of the positive polarity Pp+ is applied to the scanning electrodes Ss1, Ss2, . . . Ssm, the pre-discharge pulse of the negative Pp- is applied to the sustaining electrode Su and, after discharge has occurred on all display cells 16, an erasing pulse Ppe is applied to scanning electrodes Ss1, Ss2, . . . Ssm to cause erasing discharge to occur to erase wall charges accumulated by the pre-discharge pulses.
During the writing period, a scanning pulse Pw is sequentially applied to each of scanning electrodes Ss1, Ss2, . . . Ssm while a data pulse Pd is selectively applied, in synchronization with the scanning pulse Pw, to the data electrode Di (1.ltoreq.i.ltoreq.n) of a display cell 16 in which a display is performed and writing discharge is allowed to occur in the cell to be used for displaying to generate wall charges.
During the sustaining discharge period, a sustaining discharge pulse of the negative polarity Psu is applied to the sustaining electrode while a sustaining discharge pulse of the negative Pss that lags 180 degrees behind the sustaining discharge pulse Psu is applied to each scanning electrode and, during the writing discharge period, necessary sustaining discharge is maintained in order to obtain desired luminance in the cell in which the writing discharge is performed.
During the sustaining charge erasing period, the sustaining discharge is erased by applying erasing pulses Pse1 and Pse2 that have narrow width and have a voltage being as low as that of the sustaining discharge pulse and pulses combined with erasing pulses Pse3 having a mild fall and a wide width and a voltage as low as that of the sustaining discharge pulse.
Conventional operations during the latter half of the sustaining discharge period and the sustaining charge erasing period disclosed in Japanese Laid-Open Patent Application No. Hei10-274955 are described hereinafter to get a clear understanding of the present invention and to show shortcomings of the conventional technologies. FIG. 14 is an expanded diagram showing waveforms appeared during the latter half of the sustaining discharge period and during the sustaining charge erasing period in an embodiment disclosed in Japanese Laid-Open Patent Application No. Hei10-274955.
Immediately before the application of a final sustaining pulse of the positive polarity Psse, negative charges produced by sustaining pulses of the negative polarity Pss and Psu are accumulated, and positive and negative charges are accumulated on a scanning electrode and on a sustaining electrode respectively.
Since negative charges on a data electrode is adapted to act so as to counteract a voltage applied at the time of writing discharge, the final sustaining pulse of the positive polarity Psse applied at a last point of the sustaining discharge period is applied in order to erase negative charges on the data electrode as well as to generate sustaining discharge.
The sustaining charge erasing period is a period to erase wall charges accumulated on each electrode during the sustaining discharge period. Wall charges on the scanning electrode and sustaining electrode are erased by pulses Pse1, Pse2 and Pse3. Wall charges on the data electrode are erased by a final sustaining pulse of the positive polarity. It is necessary that wall charges on each electrode do not exist after the sustaining charge erasing period and that the discharging cell is electrically neutral.
When the final sustaining pulse of the positive polarity is applied, since the positive charge on the scanning electrode is superposed on the negative charge on the data electrode, an effective voltage in the discharge gas space exceeds an opposing discharge starting voltage, and further due to superposition of negative charges on the sustaining electrode, the effective voltage exceeds a surface discharge starting voltage. The opposing discharge refers to discharge occurred between the scanning and data electrodes, or between the sustaining and data electrodes. Therefore, by the application of the pulse Psse, both the surface discharge and the opposing discharge occur at the same time.
As depicted in FIG. 11, a phosphor is provided in a layer adjacent to the gas discharge space 8 on the data electrode 7 and a protecting layer is provided in a layer adjacent to a gas discharge space 8 on the scanning electrode 3 and the sustaining electrode 4. A substance having a large secondary emission coefficient such as magnesium oxide or the like is used as a material for the protecting layer 13. Accordingly, in the case of discharge where the scanning electrode or the sustaining electrode is used as a cathode, since the secondary emission of the cathode is large and the discharge starting voltage is low, the growth of the discharge is rapid. On the other hand, in the case of discharge where the data electrode is used as a cathode, the secondary emission is small and the discharge starting voltage is high.
If the surface discharge and the opposing discharge in which the data electrode is used as a cathode occur by the application of the pulse Psse, the opposing discharge that cannot grow solely to be strong grows and becomes strong due to active particles generated by the surface discharge in the discharge gas space. By the effect of the strong opposing discharge, the surface discharge becomes much stronger. That is, if the opposing discharge by the final sustaining pulse of the positive polarity and the surface discharge occur at the same time, both of them interacts with each other.
Accordingly, if the surface discharge and the opposing discharge occur at the same time, since the surface discharge is the sustaining discharge, the discharge state varies depending on the amount of display load which exerts an influence on the opposing discharge and, as a result, negative charges on the data electrode cannot be erased and, reversely, excessive positive charge is accumulated, causing the data electrode not to be electrically neutral. If many residual negative charges stay on the data electrode, the internal voltage counteracts an external voltage at the time of the writing discharge, the effective voltage in the discharge gas space is lowered and the writing discharge of a selected cell does not occur. If many residual positive charges still stay on the data electrode, the internal voltage is superposed on the external voltage generated by the scanning pulse or sustaining pulse of the negative polarity, causing discharge to occur in a non-selected cell.
Thus, in the conventional technology as disclosed in Japanese Laid-Open Patent Application No. Hei10-274955, if the opposing and surface discharges by the final sustaining pulse of the positive polarity occur at the same time, control on charges on the data electrode becomes difficult, presenting a problem in that a malfunction during the writing period and sustaining period occurs.